TAKAYAMA, JAPAN--A team of Japanese and American physicists at the Neutrino '98 conference presented strong evidence today that neutrinos switch identity as they travel--something that is only possible if they have mass. Weighing only a small fraction of what an electron does, these particles may nevertheless have enough heft to shake up the Standard Model--the theory that describes the behavior of elementary particles.

Neutrinos, which swarm unnoticeably through our bodies and the entire thickness of the planet, have often been assumed to have no mass. But over the years, physicists studying neutrinos produced by the sun, the upper atmosphere, and nuclear reactors and particle accelerators have often found fewer than expected. One possible explanation is that neutrinos undergo a kind of gradual identity crisis as they travel, slowly transforming among the three kinds of neutrinos. Since neutrino detectors are blind to some neutrino species, the transformed particles would escape detection. According to quantum theory, the identity change would also be a sign of mass.

The new results come from the Super-Kamiokande detector, a 12.5-million-gallon tank of water buried deep within the Japanese Alps. The detector spots neutrinos produced by cosmic rays in the upper atmosphere as they crash into water molecules in the tank. If the neutrino is a muon type, the collision spawns a particle called a muon. Electron type neutrinos produce electrons. As the electrons and muons speed through the tank, they produce a flash of light that reveals the direction they came from. The team had already found fewer muon neutrinos than theory predicts. Now they have shown that the deficit varies with angle, with fewer muon neutrinos coming up through the Earth or from the horizon than down from overhead. That's strong evidence for neutrino mass, they say, because the odds of an identity switch are predicted to vary as the neutrinos travel longer distances.

"People will now have to take the idea of neutrino mass more and more seriously," says Eric Norman, a physicist at the Lawrence Berkeley National Laboratory in California. The findings still need to be confirmed by a different type of experiment, perhaps one using neutrinos from an accelerator, he says. But if the results hold up, they will have implications far beyond just neutrino physics--from the Standard Model, which now bestows no mass to the neutrinos, to the question of how much mass there is in the universe.